1. Field of the Invention
The invention concerns a coil system of the type having multiple of individual coils that can be activated individually for contact-free magnetic navigation of a magnetic body in a three-dimensional working space accessible in the z-direction of an orthogonal x, y, z coordinate system.
2. Description of the Prior Art
A coil system of the above type is known from “IEEE Transactions on Magnetics”, Vol. 32, No. 2, Mar. 1996, pages 320 through 328.
In medicine, endoscopes and catheters are used that are inserted via incisions or body openings and can be displaced from the outside of the body in their longitudinal direction, and thus are navigable only in one dimension. An optical examination is possible with light guides, whereby an endoscope tip (and therewith the viewing direction) can be panned via control wires. Additional devices, in particular for biopsy, can be placed in a working channel in the catheter. These types of probes, however, are navigable only in a limited manner, particularly when encountering branches that are located at points further removed from a body opening. The ability to apply a contact-free force to assist navigation in such situations would expand the applications in which such devices could be used.
A magnetic coil system for a contact-free magnetic probe control is known from the aforementioned publication as well as U.S. Pat. No. 5,125,880. Such a magnetic coil system has six (advantageously superconducting) individual coils that are arranged on the faces of a cube whose position can be mathematically described in an orthogonal x, y, z coordinate system. Variable field directions and field gradients are generated with these coils in order to direct or to move a catheter with magnetic material or magnetic implants into a (for example human) body to be examined for therapy purposes. Unlimited navigation freedom of the magnetic body, however, cannot be achieved with a magnetic coil system composed of six individual coils.
The generation of magnetic field gradients is known from MRI (magnetic resonance imaging) systems for medical diagnostics. A coil system for this purpose is known, for example, from DE 39 37 148 C2.
It is also known to utilize such field gradients for determination of the momentary position and alignment of an object (such as, for example, a catheter) in a three-dimensional working space (such as, for example, a human body). A corresponding apparatus is known from WO 00/13586 A. For this purpose, the apparatus has a field generator for generation of MRI gradient fields with which electrical voltages are induced in sensor coils of the object. These electrical voltages are then relayed to signal processing electronics via a conductor system connected with the object. Contact-free, magnetically controllable movement of the object, however, is not possible.
A magnetic coil system with three coils is described in U.S. Pat. No. 6,241,671, and U.S. Pat. No. 6,529,761 discloses an arrangement of a few permanent magnets arranged such that they can rotate around a patient, the field of these permanent magnets being influenced by magnetic screens and that can generate a magnetic wave for movement of a magnetic probe.
Magnetic coil systems with rotatable permanent magnets for controlling magnetic catheters, in particular under x-ray monitoring, are also known. Nothing is said in this prior art about methods for a position stabilization of magnetic probe bodies via feedback; it is assumed that a magnetic probe body, specified by field direction and gradient, always rests on an inner surface within a body to be examined.
A method with additional pulse coils with which a magnetic probe is moved in steps via precisely defined current pulses under computer monitoring is described in WO 96/03795 A1.
Devices known as video capsules are also known (for example from the periodical “Gastrointestinal Endoscopy”, Vol. 54, No. 1, pages 79 through 83) that serve for examination of the alimentary canal. The movement of the video capsule occurs via the natural intestinal movement, meaning that the locomotion and viewing direction are random.
A corresponding video capsule that is equipped with a rod magnet as well as with video and other intervention devices is described in DE 101 42 253 C1. An external magnetic coil system exerts forces on the rod magnet for navigation. A free-floating mode (known as a helicopter mode) with external control via a 6D mouse, feedback of force via the mouse, and position feedback via a transponder are mentioned. Details for realization of the magnetic coil system and for operation of its individual coils are not provided.
A magnetic coil system for contact-free navigation or movement of a (ferro)magnetic body (such as, for example, a rod magnet) in a working space is proposed in unpublished DE patent application 103 40 925.4 from 5 Sep. 2003. The body is aligned in the working space and/or a force is to be exerted on the body with this magnetic system. The alignment as well as the magnitude and direction of the force on the body are magnetic and can be predetermined from the outside without a mechanical connection. For this purpose, a three-dimensional working space is assumed that is surrounded by surfaces stretching in a right-angled x, y, z coordinate system. The coil system has fourteen individually controllable individual coils that are fashioned for generation of the three magnetic field components Bx, By, Bz as well as the generation of five magnetic field gradients from the symmetrical (with regard to its diagonal D) gradient matrix
            (                                                                  ⅆ                                  B                  x                                                            ⅆ                x                                                                                        ⅆ                                  B                  y                                                            ⅆ                x                                                                                        ⅆ                                  B                  z                                                            ⅆ                x                                                                                                        ⅆ                                  B                  x                                                            ⅆ                y                                                                                        ⅆ                                  B                  y                                                            ⅆ                y                                                                                        ⅆ                                  B                  z                                                            ⅆ                y                                                                                                        ⅆ                                  B                  x                                                            ⅆ                z                                                                                        ⅆ                                  B                  y                                                            ⅆ                z                                                                                        ⅆ                                  B                  z                                                            ⅆ                z                                                        )        ↘                                                    D        ↘          ,whereby two of the diagonal elements of the gradient matrix and one of the outer diagonal elements from the three symmetrical (relative to the diagonal) gradient element pairs of the gradient matrix are to be generated with the individual coils.
In the proposed magnetic coil system surrounding the working space like a cage, it is assumed that, due to the conditions rotH=0 and divB=0 imposed by the Maxwell equations—whereby the quantities indicated in bold print symbolize vectors, only three independent gradients of the possible six field gradients dBx/dy, dBx/dy, dBy/dx, dBy/dz, dBz/dx and dBz/dz must be generated and only two of the three field gradients dBx/dx, dBy/dy and dBz/dz. Eight different current patterns (corresponding to the eight magnetic degrees of freedom) must be able to be impressed on the fourteen individual coils with currents of equal magnitude. These current patterns each predominantly generate a field component or a field gradient. Every combination of magnetic field components and field gradients allowable according to the Maxwell equations can then be generated by superimposition.
A contact-free control/movement (=navigation) of a magnetic body in the sense of a (mechanical) contact-free alignment of this body and/or of a force exertion on this (for example a probe connected with a magnetic element such as, for example, a catheter, endoscope or a video capsule according to DE 101 42 253 C1) by means of magnetic fields in a working space is enabled in this manner.
The following design features can additionally be provided individually or in combination with one another for the proposed magnetic system.                The fourteen individually controllable individual coils can thus be arranged on paired surfaces situated opposite one another and at least one tube-shaped generated surface extending in the z-direction. The faces of a cuboid or cube can thereby be spanned up to the generated surface. They do not necessarily need to be fashioned flat.        At least six of the individual coils can thereby lie on the paired, opposite frontal surfaces or lateral surfaces of the working space and serve for generation of the three magnetic field components Bx, By, Bz as well as the two diagonal elements of the gradient matrix. At the same time at least four of the individual coils can be arranged distributed on the at least one tube shaped generated surface surrounding the working space (as viewed in the circumferential direction) and serve for generation of at least one extra-diagonal element of the gradient matrix. The required three extra-diagonal elements can thus be formed together with the remaining individual coils.        For this purpose,six of the individual coils can be situated as three coil pairs on the paired, opposite frontal or, respectively, lateral surfaces of the working space andeight of the individual coils form two coil arrangements that are situated one after another (viewed in the z-direction) on the at least one tube-shaped generated surface, and whose respective four individual coils are arranged distributed on the generated surface (viewed in the circumferential direction) and serve for generation of the three extra-diagonal elements of the gradient matrix.        Instead of this, in the proposed coil system a coil pair of individual coils can lie on the frontal surfaces of the working space and serves for generation of the magnetic field component Bz as well as the diagonal element dBz/dz of the gradient matrix,a respective coil arrangement can be composed of respectively two individual coils arranged one after another (viewed in the z-direction) lies on the paired, opposite lateral surfaces and serves for generation of the magnetic field component Bx or, respectively, By,a coil arrangement made in particular of four individual coils can be distributed (as viewed in the circumferential direction) on the at least one tube-shaped generated surfaceandthe coil arrangement on the lateral surfaces and the generated surface can serve for generation of a further diagonal element and the three extra-diagonal elements of the gradient matrix.        The at least one generated surface can be located within the inner space spanned by the six paired, opposite surfaces.        Given the embodiments rendered in the preceding the field gradient coils situated on the (imaginary) generated surface are designed saddle-shaped. It is thereby possible that their frontal arc segments running in the circumferential direction on the generated surface lie next to one another (as viewed in the circumferential direction), i.e. respectively occupy an arc angle of >90° or overlap.        Moreover, at least some of the field component coils can be designed as planar rectangular coils or circular coils.        Coil pairs and/or coil arrangements can respectively be formed from individual coils with identical shape.        The coil pairs composed of individual coils can be arranged orthogonal to one another for generation of the magnetic field components.        Parts made from magnetically soft material can also be associated with the outside of the coil system for field amplification and/or field shielding.        Means for detection of the position of the magnetic body within the working space can also be provided.        In general a computer is used to control the fourteen individual coils of the magnetic coil system in that it controls their respectively associated current feed dependent on the respective position of the magnetic body to be moved.        
In the proposed magnetic coil system a circular cylinder formed by one of eight saddle coils is thus enclosed by a cuboid comprising six Helmholtz coils.